Melanin is the natural pigment synthesized in highly specialized cells, melanocytes, present in the skin. The primary function of the endogenous melanin is photoprotection of skin and eye tissues against the harmful action of UV light. Overproduction of the melanin pigment is characteristic of melanoma, a form of skin tumor. This research project has three main objectives: to study molecular mechanisms of the melanocytotoxic action of phenolic thioether amines (PTEA); to study free radical processes induced by UV irradiation of pheo- and eumelanin pigments and melanogenic compounds cysteinyldopas and hydroxyindoles; to study electron-transfer properties of melanins and melanin precursors. We found that incubation of depigmenting PTEA agents 4-S-cysteinylphenol (4-S-CP), 4-S-cysteaminylphenol (4-S-CAP), 4-S-homo-cysteaminylphenol (4- S-homo-CAP) and N-acetyl-4-S-cysteaminyl phenol (N-Acetyl-4-S.CAP) in aerated aqueous buffer (pH 6) in the presence of mushroom tyrosinase and metal ions (Mg2+, Zn2+) produced ortho-semiquinone radical/metal ion complexes. In the absence of the metal ions the formation of free radicals was observed only from 4-S-CP and 4-S-CAP. Other PTEA agents afforded weak, melanin-like EPR signals. UV photolysis (>300 nm) of by- products of melanin biosynthesis 5-hydroxy-6-methoxyindole-2-carboxylic acid and 5-methoxy-6-hydroxyindole-2-carboxylic acid in aqueous solutions (pH 10) generated indoloxyl free radicals, hydrogen atoms and/or hydrated electrons. The electron transfer capacity of synthetic DOPA-melanin and melanin precursors (tyrosine, phenol, DOPA, catechol, 6-hydroxy-dopamine) was tested using arenediazonium compounds as electron acceptors. Melanin, catechol, DOPA and 6-hydroxy-dopamine efficiently reduced these diazonium cations which resulted in the formation of the highly reactive aryl radicals. Because aryl radicals are known to add to adenosine and/or guanosine residues in DNA, this type of reductive activation of diazonium compounds may be related to their metagenic and carcinogenic activities.